REFACTOR: remove the shell commands (#8415)

Related to #8368.

# Description
as planned in #8311, the `enter`, `shells`, `g`, `n` and `p` commands
have been re-implemented in pure-`nushell` in the standard library.
this PR removes the `rust` implementations of these commands.

- all the "shells" tests have been removed from
`crates/nu-commnand/tests/commands/` in
2cc6a82da6, except for the `exit` command
- `cd` does not use the `shells` feature in its source code anymore =>
that does not change its single-shell behaviour
- all the command implementations have been removed from
`crates/nu-command/src/shells/`, except for `exit.rs` => `mod.rs` has
been modified accordingly
- the `exit` command now does not compute any "shell" related things
- the `--now` option has been removed from `exit`, as it does not serve
any purpose without sub-shells

# User-Facing Changes
users may now not use `enter`, `shells`, `g`, `n` and `p`
now they would have to use the standard library to have access to
equivalent features, thanks to the `dirs.nu` module introduced by @bobhy
in #8368

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- ⚫ `toolkit test`
- ⚫ `toolkit test stdlib`

# After Submitting
the website will have to be regenerated to reflect the removed commands
👍

crates/nu-std/lib/dirs.nu

@@ -27,6 +27,8 @@ export def-env "dirs add" [
     _fetch 0
 }
 
+export alias enter = dirs add
+
 # Advance to the next directory in the list or wrap to beginning.
 export def-env "dirs next" [
     N:int = 1   # number of positions to move.
@@ -34,6 +36,8 @@ export def-env "dirs next" [
     _fetch $N    
 }
 
+export alias n = dirs next
+
 # Back up to the previous directory or wrap to the end.
 export def-env "dirs prev" [
     N:int = 1   # number of positions to move.
@@ -41,6 +45,8 @@ export def-env "dirs prev" [
     _fetch (-1 * $N)    
 }
 
+export alias p = dirs prev
+
 # Drop the current directory from the list, if it's not the only one.
 # PWD becomes the next working directory
 export def-env "dirs drop" [] {
@@ -54,6 +60,8 @@ export def-env "dirs drop" [] {
     _fetch 0
 }
 
+export alias dexit = dirs drop
+
 # Display current working directories.
 export def-env "dirs show" [] {
     mut out = []
@@ -67,6 +75,31 @@ export def-env "dirs show" [] {
     $out
 }
 
+export alias shells = dirs show
+
+export def-env "dirs goto" [shell?: int] {
+    if $shell == null {
+        return (dirs show)
+    }
+
+    if $shell < 0 or $shell >= ($env.DIRS_LIST | length) {
+        let span = (metadata $shell | get span)
+        error make {
+            msg: $"(ansi red_bold)invalid_shell_index(ansi reset)"
+            label: {
+                text: $"`shell` should be between 0 and (($env.DIRS_LIST | length) - 1)"
+                start: $span.start
+                end: $span.end
+            }
+        }
+    }
+    let-env DIRS_POSITION = $shell
+
+    cd ($env.DIRS_LIST | get $env.DIRS_POSITION)
+}
+
+export alias g = dirs goto
+
 # fetch item helper
 def-env  _fetch [
     offset: int,    # signed change to position